The present invention relates generally to endoprosthesis devices, which are commonly referred to as stents, and more particularly to radiopaque stents.
Stents are generally thin walled tubular-shaped devices composed of complex patterns of interconnecting struts which function to hold open a segment of a blood vessel or other body lumen such as a coronary artery. They also are suitable for supporting a dissected arterial lining or intimal flap that can occlude a vessel lumen. At present, there are numerous commercial stents being marketed throughout the world. These devices are typically implanted by use of a catheter which is inserted at an easily accessible location and then advanced through the vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the lumen. Once in position, the stent is deployed. In the case of self-expanding stents, deployment is achieved by the removal of a restraint, such as the retraction of a delivery sheath. In the case of balloon expandable stents, deployment is achieved by inflation of a dilation balloon about which the stent is carried on a stent-delivery catheter.
The stent must be able to simultaneously satisfy a number of mechanical requirements. First and foremost, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. In addition to having adequate radial strength or more accurately, hoop strength, the stent should be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion which typically requires substantial deformation of localized portions of the stent""s structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
In addition to meeting the mechanical requirements described above, it is preferable that a stent also be fluoroscopically visible. Fluoroscopy has typically been relied upon to facilitate the precise placement of a stent as well as to verify the position of a stent within a patient throughout its service life. The use of radiopaque materials in the construction of a stent allows for its direct visualization. The most common materials used to fabricate stents are stainless steel and nickel-titanium alloys. These materials are known to be bio-compatible and satisfy the mechanical requirements for stents. However, neither of the materials is particularly radiopaque. This factor, in combination with the minimal surface area and thin wall thickness of typical stent patterns, renders stents produced from these materials insufficiently radiopaque to be adequately visualized with fluoroscopy procedures. Alternative structural materials are either excessively radiopaque or have not been proven to be sufficiently biocompatible for long term use in a vascular setting. For these reasons, simply constructing a radiopaque stent wholly out of a single material has heretofore not been considered a viable option. Thus, the art has moved in the direction of combining different materials to produce a mechanically sound, biocompatible and fluoroscopically visible stent. A number of such approaches have been developed.
One means frequently described for accomplishing flouroscopic visibility is the physical attachment of radiopaque markers to the stent. Conventional radiopaque markers, however, have a number of limitations. Upon attachment to a stent, such markers may project from the surface of the stent, thereby comprising a departure from the ideal profile of the stent. Depending on their specific location, the marker may either project inwardly to disrupt blood flow or outwardly to traumatize the walls of the blood vessel. Additionally, galvanic corrosion may result from the contact of two disparate metals, i.e., the metal used in the construction of the stent and the radiopaque metal of the marker. Such corrosion could eventually cause the marker to become separated from the stent which may be problematic should the marker be swept downstream. Finally, although such markers are typically fairly small, this approach does cause the radiopaque material to come into direct contact with living tissue which may be problematic should there be any biocompatibility issues.
Stents also have been previously marked by plating selected portions thereof with radiopaque material. However, a number of disadvantages are associated with this approach as well. This again causes the radiopaque material to come into direct contact with living tissue which, depending on the total area that is plated, can amount to a sizeable exposure. Additionally, when the stent is expanded certain portions undergo substantial deformation, creating a risk that cracks may form in the plating causing portions of the plating to separate from the underlying substrate. This has the potential for creating jagged edges that may inflict physical trauma on the lumen wall tissue or cause turbulence in the blood flowing past the stent, thereby inducing thrombogenesis. Moreover, once the underlying structural material becomes exposed, interfaces between the two disparate metals become subject to galvanic corrosion. Further, should the plating pattern cover less than all of the stent""s surfaces, the margins between the plated and unplated regions are also subject to galvanic corrosion.
As a further alternative, a stent structure has been described that is formed from a sandwich of structural and radiopaque materials. Three tubes of the materials are codrawn and heat treated to create a structural/radiopaque/structural composite. Struts and spines are then formed in the tube by cutting an appropriate pattern of voids into the tube as is well known in the art. While this approach does provide a stent that is radiopaque and that fulfills the necessary mechanical requirements, the thin cross section of the radiopaque material is nonetheless exposed along the edges of all cut lines. The biocompatiblity of the radiopaque material therefore remains an issue and more significantly, a sizeable area is thereby created that is subject to galvanic corrosion. Any cuts in the sandwich structure cause two disparate metal interfaces, i.e., the juncture between the outer structural layer and the central radiopaque layer as well the juncture between the central radiopaque layer and the inner structural layer, to become exposed to blood, an electrolytic solution.
As can be seen, composite stents, whether of the plated or coated type, sandwich type, or simply those equipped with markers, have several disadvantages; namely, potential flaking of the radiopaque coating or plating, galvanic corrosion, or poor biocompatibililty. Thus, a stent configuration is required that overcomes the shortcomings inherent in previously known devices. Preferably, such a stent would be biocompatible, possess the required mechanical characteristics, would be sufficiently radiopaque to be readily visible using fluoroscopy procedures, and would be formed from a single material.
The present invention provides a stent made from a single material that overcomes the shortcomings of previously known stent devices. The stent fulfills all of the mechanical and structural requirements attendant to its function as a stent. Moreover, in contrast to the prior art, at least a portion of the stent is fluoroscopically visible without the addition of an extra layer of radiopaque material.
The stent of the present invention includes generally a plurality of cylindrical rings that are interconnected by a plurality of links. The stent""s advantages are achieved by incorporating high metal mass density into portions of the stent such as into selected connecting links which possess sufficient mass to be readily visible using typical fluoroscopy procedures. In order to avoid the difficulties associated with machining differential wall thickness stents, in an exemplary embodiment, the stent has a uniform wall thickness and the selected high-mass links are formed as circular disks or as other shapes with a comparatively large surface area with respect to the narrow rectangular surfaces of the standard connecting links and cylindrical rings which form the body of the stent.
Each of the cylindrical rings making up the stent have a proximal end and a distal end and a cylindrical plane defined by a cylindrical outer wall surface that extends circumferentially between the proximal end and the distal end of the cylindrical ring. The cylindrical rings are interconnected by at least one standard or high-mass connecting link which attaches one cylindrical ring to an adjacent cylindrical ring. Generally, at least every other ring will include at least one high-mass link. In addition, each high-mass link is circumferentially offset from the previous high-mass link in a preceding ring to establish a spiral pattern of high-mass links around the periphery of the stent. The spiral pattern of high-mass links renders the stent fluoroscopically visible regardless of the stent""s orientation within a body lumen.
The cylindrical rings typically comprise a plurality of alternating peaks and valleys, where the valleys of one cylindrical ring are circumferentially offset from the valleys of an adjacent cylindrical ring. In this configuration, the connecting links attach each cylindrical ring to an adjacent cylindrical ring so that the links are positioned substantially within one of the valleys and attach the valley to an adjacent peak.
While the cylindrical rings and connecting links generally are not separate structures, they have been conveniently referred to as rings and links for ease of identification. Further, the cylindrical rings can be thought of as comprising a series of U""s, W""s and Y-shaped structures in a repeating pattern. Again, while the cylindrical rings are not divided up or segmented into U""s, W""s and Y""s, the pattern formed by the rings resembles such a configuration. The U""s, W""s and Y""s promote flexibility in the stent primarily by flexing and by tipping radially outwardly as the stent is delivered through a tortuous vessel.
The connecting links are positioned so that they are within the curved part of a U-shaped portion. Where the connecting link is a high-mass link, the U-shaped portion broadens to form a generally semicircular shape. Positioning the connecting links within a U-shaped portion generally increases the amount of vessel wall coverage, and where a high-mass link is used, vessel wall coverage is further increased locally due to the large surface area of the high-mass link. Since the connecting links do not substantially deform (if at all) when the stent is expanded, the selected use of high-mass links does not increase the balloon pressure required to expand a balloon expandable embodiment of the stent, nor do the high mass links increase the spring tension required to expand a self-expanding embodiment of the stent.
The number and location of high-mass connecting links can be varied as the application requires. Since the high-mass links typically have the same thickness as the standard connecting links, substituting standard links with high-mass links has very little impact on the overall longitudinal flexibility of the stent. Thus, in typical stent positioning applications the stent may have only one high-mass link between every other cylindrical ring. In applications, where a high degree of radiopacity is desired, the stent may have multiple high-mass links between each adjacent cylindrical ring.
In one embodiment of the invention, selected portions of the rings and links are of a higher metal mass density than the remaining portions of the stent. So for example, a portion of the stent that would have a spiral shape if traced around the circumference of the stent would have a higher metal mass density than the remaining portions of the stent. When the stent is oriented in a vessel, the higher mass will be readily visible under fluoroscopy. Also, since the higher mass material is in a spiral shape, it will be much easier to determine the position of the stent with respect to the vessel and to more easily determine whether the stent is fully expanded and implanted in the vessel.
The stent may be formed from a tube by laser cutting the pattern of cylindrical rings and connecting links in the tube. The stent also may be formed by laser cutting a flat metal sheet in the form of the cylindrical rings and links, and then rolling the pattern into the shape of the tubular stent and providing a longitudinal weld to form the stent.